1. Field of the Invention
The present invention relates to a circuit for discriminating dual tone multiple frequency signals (referred as DTMF signals) and, more particularly to a circuit for discriminating the DTMF signals for a remote-controlled telephone apparatus.
2. Description of the Prior Art
Recently, there have been developed many varieties of remote-control systems using the telephone line. In some such systems, for example, an automatic telephone answering apparatus may perform many useful functions, such as sending prerecorded answering messages or outgoing messages signals (referred as OGM signals hereafter) to callers on remote telephone sets, and for recording an incoming message signal (referred as ICM signal hereafter) from callers. However, an automatic telephone answering apparatus may include additional functions responsive to the remote control operation through the telephone line. In such an automatic telephone answering apparatus, for example, an owner or a subscriber of the automatic telephone answering apparatus is able to operate his or her automatic telephone answering apparatus from outside through a remote telephone set, so as to play back recorded ICM signals from callers in his or her absence, or to record new OGM signals or renew the OGM signals. The remote control operation is usually performed by sending specific secret or personal codes, e.g., the DTMF signals through a remote telephone set. The DTMF signals correspond to each of twelve button codes of a standard telephone set, i.e., "0" to "9", "*" and "#".
In the remote control operation, first a subscriber calls his or her automatic telephone answering apparatus through a remote telephone set by dialing. The automatic telephone answering apparatus activates its own internal circuit in response to a calling signal from the remote telephone set. The subscriber then sends some DTMF signals to the automatic telephone answering apparatus by operating buttons of the remote telephone set. The automatic telephone answering apparatus is provided with a processor, such as a microcomputer, for controlling the remote control operations and other necessary controls in response to the received DTMF signals. The microcomputer identifies or discriminates the DTMF signals and then performs a prescribed control corresponding to the received DTMF signals.
Normally each DTMF signal comprises a specific combination of two signals, one selected from a group of four low frequency signals and the other from a group of three high frequency signals. The four low frequency signals typically are comprised of a 697 kHz signal, a 770 kHz signal, an 852 kHz signal and a 941 kHz signal, while the three high frequency signals are comprised of a 1,209 kHz signal, a 1,336 kHz signal and a 1,477 kHz signal. The twelve telephone buttons "0" to "9", "*" and "#" correspond to the four low frequency signals and the three high frequency signals in a matrix circuit, as shown in FIG. 1. Thus, when the telephone button "1", for example, is operated, a DTMF signal with a combination of the 697 kHz low frequency signal and the 1,209 kHz high frequency signal is transmitted from the telephone set. The high frequency signal and the low frequency signal in the same DTMF signal should be generated within a time difference of five (5) msec. (millisecond) or less from each other when a prescribed button is operated. Also, the high frequency signal should have a sound level not more than three (3) dB lower than the sound level of the low frequency signal in the same DTMF signal.
As shown in FIG. 2, such a conventional automatic telephone answering apparatus is equipped with a line coupling transformer 20, a bell signal detection circuit 22, a line switch circuit 24, an outgoing message signal source (referred to as OGM source hereafter) 26 such as a magnetic tape apparatus, an incoming message recorder (referred to as ICM recorder hereafter) 28 such as a magnetic tape apparatus and a circuit 30 for discriminating the DTMF signals. In FIG. 2, the conventional DTMF signal discriminating circuit 30 comprises a selective signal coupling circuit 32, a frequency signal extracting circuit 34 and a microcomputer 36. The line coupling transformer 20 is coupled to a telephone line TL through the line switch circuit 24. The line switch circuit 24 has a control terminal 24a connected to the microcomputer 36. The bell signal detection circuit 22 is coupled between a primary winding 20a of the line coupling transformer 20 and the microcomputer 36 for detecting a bell signal through the telephone line TL. The microcomputer 36 activates the line switch circuit 24 in response to a detection signal from the bell signal detection circuit 22. The OGM source 26 and the ICM recorder 28 are arranged so that the OGM source 26 generates an OGM signal and the ICM recorder 28 records an ICM signal under the control of the microcomputer 36 in a prescribed automatic telephone answering mode.
In the DTMF signal discriminating circuit 30, the selective signal coupling circuit 32 has an input terminal 32a, an input/output terminal 32b and an output terminal 32c. The input terminal 32a is provided for receiving the OGM signal applied from the OGM source 26. The input/output terminal 32b receives the ICM signal applied from a remote telephone set through the telephone line TL and the line coupling transformer 20. The output terminal 32c is connected to the microcomputer 36 through the frequency signal extracting circuit 34. The frequency signal extracting circuit 34 is comprised of first to seventh band pass filters (referred to as BPF circuits hereafter) 38a, 38b, 38c, . . . 38g, and also first to seventh phase locked loop circuits (referred to as PLL circuits hereafter) 40a, 40b, . . . 40g. The first to seventh BPF circuits 38a, 38b, 38c, . . . 38g are connected in parallel between the selective signal coupling circuit 32 and the microcomputer 36. That is, input terminals of the first to seventh BPF circuits 38a, 38b, 38c, . . . 38g are connected in common to the output terminal 32c of the selective signal coupling circuit 32, while output terminals of the first to seventh BPF circuits 38a, 38b, 38c, . . . 38g are connected in parallel to first to seventh input terminals 36-Ia, 36-Ib, . . . 36-Ig of the microcomputer 36. The first to seventh PLL circuits 40a, 40b, 40c . . . 40g are connected in parallel between the first to seventh BPF circuits 38a, 38b, 38c, . . . 38g and the input terminals 36-Ia, 36-Ib, 36-Ic, . . . 36-Ig of the microcomputer 36, respectively. The first to seventh BPF circuits 38a, 38b, 38c, . . . 38g and the first to seventh PLL circuits 40a, 40b, 40c . . . 40g are responsive to the frequency signals, i.e., the signals of 697 kHz, 770 kHz, 852 kHz, 941 kHz, 1,209 kHz, 1,336 kHz and 1,477 kHz, respectively.
The operation of the conventional automatic telephone answering apparatus shown in FIG. 2, in particular, the the DTMF signal discriminating operation of the DTMF signal discriminating circuit 30 now will be described. When a subscriber calls his or her automatic telephone answering apparatus from a remote telephone set (not shown), the bell signal detection circuit 22 detects bell signals transmitted from the remote telephone set and applies a detection signal to the microcomputer 36. The microcomputer 36 then activates the line switch circuit 24 in response to the detection signal so that a communication channel is established between the remote telephone set and the automatic telephone answering apparatus. The microcomputer 36 also drives the OGM source 26 so that the OGM source 26 transmits an OGM signal prerecorded therein to the selective signal coupling circuit 32. The selective signal coupling circuit 32 is arranged so that the OGM signal applied to the input terminal 32a is selectively transmitted to the input/output terminal 32b. An ICM signal applied to the input/output terminal 32b through the line coupling transformer 20 is selectively transmitted to the output terminal 32c. Therefore, the OGM signal is transmitted to the input/output terminal 32b of the selective signal coupling circuit 32, but is prevented from being transmitted to the output terminal 32c. The audible OGM signal on the input/output terminal 32b of the selective signal coupling circuit 32 is transmitted to the remote telephone set through the line coupling transformer 20. Thus, the subscriber on the remote telephone set recognizes that the automatic telephone answering apparatus is ready to respond for remote control operations from the remote telephone set.
Referring now to FIG. 3, the operation of the prior art DTMF signal discriminating circuit 30 will be described, for example, in discriminating two DTMF signals corresponding to the button codes "1" and "3". When the subscriber sequentially operates the telephone buttons "1" and "3" for carrying out a prescribed remote control operation, first, the DTMF signal corresponding to the telephone button "1" (referred as DTMF signal "1" hereafter), is generated from the telephone set. This signal is shown by a waveform a1 in the graph A of FIG. 3, and is composed of the 697 kHz low frequency signal and the 1,209 kHz high frequency signal. Next, the other DTMF signal corresponding to the telephone button "3" (referred as DTMF signal "3" hereafter), is generated from the telephone set. This signal is shown by a waveform a2 in the graph A of FIG. 3, and is composed of the 697 kHz low frequency signal and the 1,477 kHz high frequency signal. The DTMF signals "1" and "3" are applied to the input/output terminal 32b of the selective signal coupling circuit 32 through the telephone line TL and the line coupling transformer 20. In the selective signal coupling circuit 32, the DTMF signals "1" and "3" are selectively transmitted to the input/output terminal 32b, as described above. The DTMF signals "1" and "3" are then applied to the frequency signal extracting circuit 34. In the frequency signal extracting circuit 34, the 697 kHz signal component in each of the DTMF signals "1" and "3" are extracted by the BPF circuit 38a sequentially at the times T1 and T2 corresponding to the times when the caller activates telephone buttons "1" and "3". The 1,209 kHz signal component of the DTMF signal "1" is extracted by the BPF circuit 38e at the time T1. Further the 1,477 kHz signal component of the DTMF signal "3" is extracted by the BPF circuit 38g at the time T2. The 697 kHz signal, the 1,209 kHz signal and the 1,447 kHz signal are applied to the PLL circuits 40a, 40e and 40g, respectively. The first to seventh PLL circuits 40a, 40b, 40c . . . 40g are arranged so that their outputs have a high (H) level when they are supplied with no extracted signals from their corresponding first to seventh BPF circuits 38a, 38b, 38c, . . . 38g, while their outputs have a low (L) level when they are supplied with extracted signals from their corresponding first to seventh BPF circuits 38a, 38b, 38c, . . . 38g. Thus, the output of the PLL circuit 40a has the L level twice, as shown by L level signals b1 and b2 in the graph B of FIG. 3, corresponding to receipt of the 697 kHz signal and the 1,209 kHz signal. The output of the PLL circuit 40 e has the L level once, as shown by L level signal c1 in the graph C of FIG. 3, corresponding to receipt of the 1,209 kHz signal. The output of the PLL circuit 40g also has the L level once, as shown by L level signal d2 in the graph D of FIG. 3, corresponding to receipt of the 1,477 kHz signal.
The respective outputs b1, b2, c1 and d2 of the PLL circuits 40a, 40e and 40g are applied to the input terminals 36-Ia, 36-Ie and 36-Ig of the microcomputer 36. The microcomputer 36 then discriminates the DTMF signal "1" by detecting that the output b1 of the PLL circuit 40a and the output c1 of the PLL circuit 40e both exhibit the L level at the time T1. Also the microcomputer 36 discriminates the DTMF signal "3" by detecting that the output b2 of the PLL circuit 40a and the output d2 of the PLL circuit 40g both exhibit the L level at the time T2. The periods of the DTMF signals "1" and "3", i.e., the L level periods of the outputs b1, b2, c1 and d2 of the PLL circuits 40a, 40e and 40g must be more than about thirty five (35) msec. so that the microcomputer 36 is capable of discriminating them.
This conventional DTMF signal discriminating circuit has some drawbacks. The conventional circuit may easily carry out some undesired remote control operation, without responding to the DTMF signals. In other words, the conventional DTMF signal discriminating circuit is easily influenced by undesired signals other than the DTMF signals. In an automatic telephone answering apparatus responsive to remote control, the OGM source 26 typically is used for providing callers with some subscriber's message, i.e., the OGM signal. The OGM signal is, of course, an audio frequency band signal, and the OGM signal applied to the selective signal coupling circuit 32 often leaks out to the output terminal 32c. When the leaking OGM signal includes frequency components corresponding to the specific frequency signal components of the DTMF signals, the OGM signal may be extracted by the frequency signal extracting circuit 34. As a result, the microcomputer 36 may wrongly carry out some remote control operation.
In the conventional circuit, it also is difficult to discriminate the frequency signals from the other frequency signals in the same frequency group. This is because the frequency signals in the same low or high frequency group are close in frequency to each other. Therefore, the first to seventh BPF circuits 38a, 38b, 38c, . . . 38g can easily extract the other frequency signals in the same low or high frequency group by mistake, if the first to seventh BPF circuits 38a, 38b, 38b, . . . 38g have fairly sharp frequency selection characteristics. For example, FIG. 4 shows a frequency selection characteristic diagram for both the BPF circuits 38e and 38g. In FIG. 4, the characteristics are taken from the BPF circuits 38e and 38g with their selectivities Q being set to ten (10). As shown in FIG. 4, the 1,209 kHz signal and the 1,477 kHz signal extracted by the BPF circuits 38e and 38g overlap with one another over a relatively wide frequency range. The difference in the nonoverlapped portion between these frequency ranges is not more than three (3) dB. Therefore, the microcomputer 36 may not properly discriminate between these signals.